円形

中心と周囲361個の点が必要

        private const int VERT_NUM = 362; // 円形

            vertexBuf = new VertexBuffer(typeof(CustomVertex.PositionTextured),
                VERT_NUM, device, Usage.None, CustomVertex.PositionTextured.Format, Pool.Managed); // 円形、プール自動管理
            CustomVertex.PositionTextured[] verts = new CustomVertex.PositionTextured[VERT_NUM];

            verts[0] = new CustomVertex.PositionTextured(0, 0, 0, 0, 0); // 中心
            float x, y, r = 1;
            for (int i = 0; i < VERT_NUM-1; i++)
            {
                x = (float)(r * Math.Cos((double)i/180*Math.PI));
                y = (float)(r * Math.Sin((double)i/180*Math.PI));
                verts[i+1] = new CustomVertex.PositionTextured(x, y, 0, 0, 0);
            }

            GraphicsStream gs = vertexBuf.Lock(0, 0, 0); // 全体ロック、フラグとりあえず0
            gs.Write(verts);
            vertexBuf.Unlock();

            device.DrawPrimitives(PrimitiveType.TriangleFan, 0, VERT_NUM-2); // 三角形、vertex開始点、個数